I always wondered if there was a Rev.4 board..... like you say, there is next to no information.

Comparing it with Rev.3.... looks like a few changes have taken place on the board itself (Rev.3 was notoriously buggy) - the ceramic chips you have on there are quite rare before they were replaced for the cheaper non-ceramic counterparts.

So most likely a 'fixed' board to replace Rev.3. I would imagine Rev.4 wasn't around for long.